Sildenafil (Viagra)

Sildenafil (Viagra)

Sildenafil (Viagra) is the original PDE5 inhibitor, the drug that made erectile dysfunction a treatable, unembarrassing thing. It's a short-acting, on-demand tool: you take it 30 to 60 minutes before sex, it works for roughly 4 to 6 hours, and it makes erections firmer and more reliable when there's blood flow to work with. For most men whose ED is vascular in origin (the common kind that creeps in with age, blood pressure, blood sugar, or stress), it just works, and the trial data backing that is about as strong as it gets for any drug.
It's the same class as Cialis (tadalafil), but where Cialis is the long-acting daily option, sildenafil is the sharp, predictable window. You feel nothing until you need it, you use it for a specific occasion, and there's no drug sitting in your system the rest of the time. That on-demand profile is the main reason people pick it. Beyond the bedroom it also shows up in pulmonary hypertension treatment and, in the biohacking world, as an exercise aid at altitude, more on both in the deep-dive.
For women, sildenafil is not approved for any sexual indication, but the mechanism (blood flow to genital tissue) applies the same way, and there's real trial data in specific groups. That's covered in the deep-dive too.

Deep-dive

Sildenafil blocks phosphodiesterase type 5, the enzyme that breaks down cyclic GMP. cGMP is the messenger that tells vascular smooth muscle to relax. Sexual stimulation releases nitric oxide in the penis, nitric oxide drives up cGMP, cGMP relaxes the smooth muscle, blood flows in, and you get an erection. PDE5 normally clears cGMP and ends the process. Block PDE5 and the cGMP signal lasts longer and goes further, so erections are easier to get and easier to keep. The key practical consequence: sildenafil does nothing on its own. It amplifies a signal that sexual arousal has to start. No arousal, no nitric oxide, nothing for the drug to act on.
Sildenafil is absorbed fast, reaching peak plasma levels at a median of 60 minutes (range 30 to 120 minutes) on an empty stomach, with a half-life of around 4 hours. That short half-life is the whole personality of the drug: a clean on-demand window, in and out, versus tadalafil's 17.5 hours and 24 to 36 hour coverage.
Erectile dysfunction. This is the most thoroughly studied use of any ED drug. An early systematic review and meta-analysis of 27 trials in 6,659 men found sildenafil significantly improved erectile function across every patient subgroup evaluated, and men on sildenafil were actually less likely than those on placebo to drop out of trials for any reason. A separate meta-analysis of 21 randomised trials found men were roughly 3.6 times as likely to have improved erections on sildenafil versus placebo, with a number-needed-to-treat of about two, which is an unusually strong result. It works in harder-to-treat groups too: a meta-analysis of 8 trials in 1,172 men with diabetes found it significantly improved sexual performance in diabetic ED, and a randomised trial in men with antidepressant-associated ED found 6 weeks of sildenafil improved erectile function and overall sexual satisfaction. The honest limitation across this literature: most trial participants had at least some baseline erectile function, so in men with more severe ED the real-world response rate is lower than the headline numbers suggest.
Where it doesn't help. Sildenafil doesn't touch libido, desire, testosterone, or orgasm in men. If the problem is wanting sex rather than getting an erection, this is the wrong tool, look at hormones or the psychological side instead. It also has a hard ceiling in ED caused by severe nerve damage (for example, radical prostatectomy with both nerve bundles removed), where there isn't enough nitric oxide signal for any PDE5 inhibitor to amplify. And it does nothing without arousal, which catches some people off guard, it is not an aphrodisiac, it's an amplifier.
Endothelial function and the cardiovascular angle. The same PDE5/cGMP mechanism operates in blood vessels throughout the body, which is the basis for the longevity-adjacent interest in PDE5 inhibitors. A 10-week randomised trial in 24 men with type 2 diabetes found 50 mg daily improved brachial artery flow-mediated dilation, a direct measure of how well blood vessels dilate, alongside improved erectile rigidity. Acute dosing studies have shown sildenafil dilates coronary arteries and prolongs flow-mediated dilation in the brachial artery while also mildly inhibiting platelet activation. This is real, but it's mostly short-term and mechanistic, and the effect is less convenient to chase with sildenafil than with daily tadalafil simply because of the dosing schedule. Nobody has run a long-term cardiovascular outcomes trial on sildenafil for this purpose. If endothelial health is the goal, the daily-dosing logic points to tadalafil rather than sildenafil.
Altitude and exercise. Sildenafil relaxes the pulmonary arteries, which is why it's used in pulmonary arterial hypertension and why it gets attention from climbers and endurance athletes. At altitude, low oxygen constricts the pulmonary blood vessels and raises the pressure the right side of the heart has to pump against, which limits exercise capacity. In a randomised crossover trial at Mount Everest base camp, sildenafil reduced hypoxic pulmonary hypertension and increased maximum exercise capacity both at simulated low-altitude hypoxia and at real high altitude. A 6-day placebo-controlled study at 4,350 m found it largely normalised the altitude-driven rise in pulmonary artery pressure. The important caveat: this benefit appears specific to hypoxia. A trial in trained men found sildenafil improved cardiac output and performance during hypoxic exercise but did nothing in normoxia, and a time-trial study at simulated moderate altitude found no performance benefit at all. So: a plausible tool for unacclimatised exertion at genuine altitude, not a sea-level ergogenic aid, and response varies a lot between individuals.
Pulmonary arterial hypertension. This is a separate, established medical use at a different dose (typically 20 mg three times daily, branded as Revatio). It's a genuine treatment, not an off-label experiment, but it's a distinct use case from the on-demand ED dose and should be managed by a specialist.
Women. Sildenafil isn't approved for any female sexual indication, and broad trials in mixed female sexual dysfunction have mostly disappointed, largely because "female sexual dysfunction" lumps together desire, arousal, and orgasm problems, and sildenafil only addresses the vascular arousal component. But when trials narrow the population to women whose problem is genital arousal and blood flow, results appear. A randomised controlled trial in 98 premenopausal women with antidepressant-associated sexual dysfunction, published in JAMA, found sildenafil significantly improved sexual function, with the clearest gains in orgasm delay and difficulty, while their antidepressant kept working. A double-blind crossover trial in 36 premenopausal women with type 1 diabetes and arousal disorder found sildenafil improved arousal, orgasm, sexual enjoyment, and pain during intercourse, with measurable increases in clitoral blood flow on Doppler. More recently, a phase 2b randomised trial of a topical sildenafil cream in 200 premenopausal women with female sexual arousal disorder found improvement in arousal sensation that was clearest in women who didn't also have orgasmic dysfunction, and in that subgroup the cream increased arousal, desire, and orgasm and reduced sexual distress. The practical read mirrors the male picture: sildenafil can help women whose dysfunction is vascular and arousal-based (diabetic, antidepressant-associated, blood-flow-limited), and is unlikely to do much when the core issue is desire. It increases genital blood flow, it does not manufacture wanting. The oral doses used in female trials run higher (50 to 100 mg) and the side effect profile, headache especially, is a real reason many women didn't love it, which is part of why topical formulations are being developed. If desire is the actual problem, PT-141 (bremelanotide) works on a completely different pathway and is the more relevant tool.

Dosage:

  • On-demand, 50 mg is the standard starting dose, taken 30 to 60 minutes before sex. Range is 25 to 100 mg. Many men land at 50 mg; 100 mg is the ceiling and more is not better, it just raises side effects. Don't dose more than once in 24 hours
  • Start at 25 mg if you're over 65, have liver or kidney impairment, or are on a strong CYP3A4 inhibitor (ketoconazole, ritonavir, itraconazole, erythromycin). Clearance is slower in these cases and the lower dose holds side effects down
  • Timing is the thing people get wrong. It needs 30 to 60 minutes to come up, and it needs sexual arousal to do anything. It is not instant and it does not work on its own
  • Effect lasts roughly 4 to 6 hours. Unlike tadalafil's 36-hour window, this is a defined session, not a weekend
  • For altitude use, the trial protocols used 50 mg, with some studies dosing before and during exposure. This is niche and the response varies a lot between people, treat it as an experiment, not a plan
  • For women, the only doses with trial support are 50 to 100 mg of the oral drug, used either on-demand or daily over 8 to 12 weeks depending on the study. There's no settled female protocol, and headache is a common enough reason for women to stop that it's worth starting at 50 mg
  • Avoid stacking with other PDE5 inhibitors (tadalafil, vardenafil, avanafil), with nitrates of any kind including amyl nitrite "poppers," or with riociguat. The nitrate interaction is genuinely dangerous and can cause a severe, sometimes fatal drop in blood pressure. This is the one hard rule
  • Forms: brand-name Viagra, cheap generic sildenafil tablets (the patent expired years ago, generics are the standard and well-controlled), and compounded chewables or troches from telehealth pharmacies. Generic tablets are the sensible default

Here's what you can expect:

Taken 30 to 60 minutes before sex on a relatively empty stomach, sildenafil makes erections firmer, easier to get, and easier to maintain, provided you're aroused. You don't feel the drug as a sensation the way you'd feel caffeine. What you notice is that things work the way they used to, and that there's a window of a few hours where that's reliable.
The most common physical signals that it's active are mild: some facial flushing, a bit of nasal congestion, maybe a light headache or warmth. Those usually show up within the first hour and mean the drug is in your system, not that something is wrong.
It does not increase desire, and it does not produce an erection without arousal. People who expect it to do the wanting for them are the ones who come away disappointed. The first time, give it a genuine 60 minutes and a low-pressure setting, performance anxiety blunts the same nitric oxide signal the drug depends on, so a tense first attempt can undersell it.
For women, expect any effect to be about arousal, lubrication, and physical genital response rather than desire, and in the daily-use trials the response built over weeks rather than landing in one dose.

Side effects & risks:

  • Headache is the most common, roughly 11 to 16% of users. Usually mild and short-lived. OTC analgesics help
  • Facial flushing from systemic vasodilation. Common, mild, harmless
  • Nasal congestion, same mechanism, same story
  • Indigestion and heartburn, fairly common, more so at 100 mg
  • Visual changes are fairly specific to sildenafil: a mild blue tint to vision, increased light sensitivity, or slight blurriness, because sildenafil weakly inhibits PDE6 in the retina. It's temporary, tracks the drug's time course, and is harmless. Sudden vision loss in one or both eyes (NAION, non-arteritic anterior ischemic optic neuropathy) is very rare but serious, and more likely in people with pre-existing optic disc crowding, smokers, diabetics, and those over 50. Stop and seek care immediately if vision suddenly drops
  • Sudden hearing loss is rare but documented, usually one-sided, sometimes with ringing. Stop and seek care if it happens
  • Priapism, an erection lasting more than 4 hours, is a urological emergency. Rare, but more likely at higher doses and in men with sickle cell disease, leukaemia, or multiple myeloma. Don't wait it out, go to the ER
  • Hypotension. Sildenafil mildly lowers blood pressure on its own. Combined with nitrates this becomes dangerous and potentially fatal. Combined with alpha-blockers (tamsulosin, doxazosin) it can cause symptomatic drops, especially when either drug is being started, space the doses and start low
  • Cardiovascular caution. Sex is mild exercise. If you have unstable angina, a recent heart attack, uncontrolled heart failure, or severe valvular disease, you shouldn't use PDE5 inhibitors until a cardiologist clears you. The concern is the exertion, not the drug itself
  • Women in the trials reported the same headache, flushing, and congestion as men, and at the 50 to 100 mg oral doses these were common enough to drive some dropouts. Sildenafil isn't recommended in pregnancy or breastfeeding, the safety data isn't there and the indications women might use it for aren't pressing during that period
  • Long-term safety at on-demand doses is well established, sildenafil has been in wide use since 1998 with no consistent signal of cumulative harm. It doesn't appear to affect testosterone, sperm, or fertility
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Blood markers

Blood pressure, baseline before starting, especially if you're on any blood pressure medication, an alpha-blocker, or anything else that lowers BP. Sildenafil is mildly hypotensive and you want a reference point. No routine rechecking is needed for occasional on-demand use; recheck if you start another BP-active drug.
Most people using sildenafil on-demand for ED need no specific bloodwork. The testing that matters is the workup behind the ED itself, not the drug. New vascular ED is a meaningful signal, so a baseline fasting glucose or HbA1c and a full lipid panel are worth doing, because ED often shows up before diabetes or cardiovascular disease announces itself elsewhere. Testosterone is worth checking if desire is low alongside the erectile issue, since that points away from sildenafil being the right fix.
Who actually needs more: anyone with known heart disease should be cleared by their cardiologist rather than screened with a blood test, and anyone on nitrates shouldn't be using the drug at all.
Prescription medication in most countries, available as inexpensive generic sildenafil.